Fluorescent lamps are well-known in the art and are used for a variety of types of lighting installations. Such lamps are characterized as low pressure discharge lamps and include an elongated envelope, whose interior surface is coated with a layer of phosphor, and an electrode at each end of the envelope. The envelope also contains a quantity of an ionizable medium such as mercury, and a starting gas at a low pressure, generally in the range of 1 to 5 mm Hg. The starting gas may consist of argon, neon, helium, krypton or a combination thereof.
One of the most commonly used methods for introducing mercury into such lamps is a mechanical dispensing unit which forms part of a so-called exhaust machine. Mercury is dispensed by the action of a slotted plunger passing through a reservoir of mercury and into the closed exhaust chamber housing the exhaust tube. The mercury falls through the exhaust tube into the lamp. This method of dispensing mercury has many drawbacks. In the first place, the mercury dispensing unit complicates the exhaust machine. In the second place, the amount of mercury introduced into the lamp envelope by this method can not be precisely controlled.
The lamp during processing is at a high temperature and is in open communication with the exhaust machine. As a result, it is inevitable that a portion of the introduced mercury evaporates and disappears from the lamp, or a portion of the filling gas is driven out of the lamp. Furthermore, the introduction of mercury through the exhaust tube involves the risk of mercury getting stuck in the exhaust tube so that after lamp sealing, the lamp contains too little or no mercury at all. For these reasons a large excess of mercury, namely a multiple of the quantity required by the lamp is generally introduced. Finally, working with mercury on the exhaust machine requires additional safety precautions on medical grounds.
An alternative method of dispensing mercury is to place inside the lamp a mercury compound that is inert under lamp processing conditions but can later be activated to release mercury. Disadvantageously, this method releases impurities, which then require special gettering. Moreover, this method requires a relatively long period of time to activate the mercury compound (e.g., 20 to 30 seconds). As a result, this method of dispensing mercury does not readily lend itself to high speed production machinery.
Another method of introducing mercury into an arc discharge lamp is set forth in U.S. Pat. No. 4,553,067 which issued to Roche et al on Nov. 12, 1985 and is assigned to the same Assignee as the present Application. Therein a mercury dispensing target is located within an exhausted lamp having a coil at each end of the lamp. The dispensing target is affixed to a lead-in wire adjacent one of the coils. During processing, the mercury target is heated by bombarding the target with a directed stream of electrons produced by one of the coils which causes the contained mercury to be released. Although this method reduces mercury release time to 3.5 seconds, it is desirable to obtain further reductions.
U.S. Pat. No. 4,870,323, which issued to Parks, Jr. et al on Sept. 26, 1989 and is assigned to the same Assignee as the present Application, describes a method of dispensing mercury into a fluorescent lamp wherein portions of the mount structure are coated with an insulating coating (e.g., zirconium dioxide) to decrease the time needed to release mercury from a capsule within the lamp. A directed stream of electrons is focused to a portion of the mercury dispensing capsule devoid of the insulating coating. Although this method is effective in reducing the mercury release time, the application of an insulating coating to the various portions of the mount structure may be impractical in commercial production.